The present invention relates to a spraying device for the electrostatic spraying of powdered or liquid coating material on articles and, more particularly, to a spraying device of this type at least a portion of which is constructed of electrically conductive material for forming a charging electrode. The charging electrode is connected to a high-voltage generator, disposed axially rearward of the charging electrode.
Generally, a spraying device of the type referred to herein includes an elongate, axially extending, support that is constructed of electrically insulated material, a spraying mechanism supported on a front end of the elongate support, and a protective tube constructed of electrically insulating material surrounding at least part of the spraying mechanism and the front and central portions of the elongate support. The rear of the elongate support protrudes from the rear of the protective tube and a spray head component of the spraying mechanism protrudes from the front of the protective tube.
A spraying device of the above mentioned type is shown in French Patent Application No. 2 543 853. But the spraying device must be connected to an external high-voltage generator, which generator is typically shared by several spraying devices. A rotary spray head is also included for spraying liquid coating material radially relative to the axis of the spraying device.
U.S. Pat. No. 3,731,145 discloses a pistol-shaped spraying device for spraying liquid coating material from a spray nozzle attached to a stationary spray head. A high-voltage generator which is powered from an external low voltage DC source is disposed within the unit. Adjustment of the magnitude of the high voltage is possible.
A similar pistol-shaped spraying device for powdered--rather than liquid--coating material is described in U.S. Pat. No. 3,608,823 which is based on Federal Republic of Germany Pat. No. 20 65 699.
A liquid spraying device with a rotary, pneumatically driven, spray head is described in U.S. Pat. No. 3,281,076. The rotary spray head is electrically conductive, bell, cup, disc or similarly shaped, forms an electrode for the electrostatic charging of the material being sprayed, and powered from an external high-voltage generator. Typically, a spray-coating system is formed which includes several such spray heads all of which are connected to a common external high-voltage generator. This requires a high-voltage generator which is very large and powerful, prevents local adjustment of the high voltage within each spray head and requires the use of expensive high-voltage cables to provide a connection to each spray head. Moreover, flashing over of the high voltage to the housing is possible and extremely risky to personnel operating such systems.
High-voltage generators are described in the aforementioned U.S. Pat. Nos. 3,731,145 and 3,608,823, the contents of which are incorporated by reference herein. Typically, a high-voltage generator includes at least one voltage multiplier circuit, preferably comprising a cascade circuit of capacitors and rectifiers or a voltage divider circuit with ohmic resistors. The voltage multiplier circuit transforms a low alternating voltage of, for instance, 4000 V to a voltage of, for example, 140,000 V. Often, a transformer for transforming an input alternating voltage of, for example, 10 V to the required 4000 V is also incorporated in the high-voltage generator. In the latter case, thinner and less expensive electrical feed cables can be used.
It is also known to include in high-voltage generators DC to AC converters, normally comprised of an oscillator for producing a 10 V alternating voltage from a direct voltage whereby the feed lines to the spraying device may be constituted of a simpler DC cable which poses no danger to users. All of the foregoing techniques may be applied in the context of the present invention.